On what Prime Minister Jacinda Ardern labeled as "one of New Zealand's darkest days," 49 people were killed in shootings at two mosques in the city of Christchurch, the worst act of violence in the country in nearly three decades.

“It is clear that this can now only be described as a terrorist attack,” Ardern said.

Police arrested three men and a woman. One of the suspects has been charged with murder.

New Zealand Police Commissioner Mike Bush said other suspects could be involved. Police are not assuming the incident was contained to the city, he said.

"Let's not presume the danger is gone," he said, adding that police had also defused a number of improvised explosive devices found on vehicles after the mosque shootings.

A man who claimed responsibility for the shootings left a 74-page anti-immigrant manifesto and said he was a 28-year-old white Australian and a racist. Authorities did not say who they detained, but said none had been on any watch list.

Australian Prime Minister Scott Morrison confirmed that one of the four people detained was an Australian-born citizen.

It is the deadliest shooting in New Zealand since 1990, when David Gray killed 13 people before being shot and killed by police in the town of Aramoana, The Guardian reported.

Police confirmed two shooting locations: One at Linwood Avenue and one at Deans Avenue in the city.

"Clearly, what has happened here is an extraordinary and unprecedented act of violence," Ardern said. She said more than 20 people were seriously wounded.

She said of the arrested: "These are people who I would describe as having extremist views that have absolutely no place in New Zealand and in fact have no place in the world."

Ardern said that authorities did not have any reason to believe there are other suspects, but were not assuming that, and that the national security threat level was lifted from low to high.

She said many of the people affected by the atrocity were immigrants who "were parts of communities that they loved and who loved them in return."

She voiced "the strongest possible condemnation of the ideology of the people who did this. You may have chosen us – we utterly reject and condemn you."

Iman Atta of Tell MAMA, a British organization that supports victims of anti-Muslim prejudice, said in a statement: "Anti-Muslim hatred is fast becoming a global issue and a binding factor for extremist far right groups and individuals. It is a threat that needs to be taken seriously."

A man dressed in black entered the Masjid Al Noor mosque on Deans Avenue at about 1:45 p.m., witness Len Peneha said. Then, Peneha heard dozens of shots, followed by people running from the mosque.

The reported shooter fled the mosque and dropped what appeared to be a semi-automatic weapon in Peneha's driveway, he said. Peneha went into the mosque to try to help, he said.

"I saw dead people everywhere. There were three in the hallway, at the door leading into the mosque, and people inside the mosque," he said. "I don't understand how anyone could do this to these people, to anyone. It's ridiculous."